Dead Bird Quiz answers. Finally. After haranguing you.

18 09 2013

Thanks to those of you who indulged me and my shrieking for responses to this latest DBQ. I got several responses for Bird A, all identifying it as a tern, and some specifying Common Tern. I am relieved to concur with you all. There are many live Common Terns swooping and feeding at MA_23, but of course, the presence of live birds is not always at all predictive of what might turn up dead. In this case, however, it appears that one of those terns didn’t make it and washed up at the river’s edge.

How to make the i.d.? Well, it’s terny. Terns have long, pointy wings, a thin, pointy beak, and, for many species, a forked tail. Among the terns with all these features, the i.d. generally hinges on the color pattern on the head, the relative lengths of the wings and tail, and the depth of the tail fork. Out Bird A has a black cap but a white forehead, gray wings with darker bars on the upperwing, and a fairly deeply forked tail. The tail itself is mostly white with dark outer margins. Many of these things help differentiate our Bird A from the other tern species with which it is most likely to be confused.

A black cap with a white forehead is also present in Arctic and Roseate Terns. Roseates, however, have long tails that usually project past the wings when the bird is laid out. The tail is all white in Roseates, and our Bird A has those dark margins. The measurements help us a good deal too–both the wing and tarsus are substantially shorter in Roseates than in Commons. How about Arctic Terns? They can be similar to Common Terns in many respects. One key is to compare the color of the secondaries. In Arctic Terns, the secondaries are white, whereas our Bird A has dark secondaries. Tarsus is quite a bit shorter in Arctic Terns than Common Terns.

This is a young Arctic Tern. Look how stubby their legs are! (photo by David Blaikie)

This is a young Arctic Tern. Look how stubby their legs are! (photo by David Blaikie)

Two breeding Common Terns. The one in back is farther along in the transition to basic (non-breeding) plumage. (photo by Jeff Lewis)

Two breeding Common Terns. The one in back is farther along in the transition to basic (non-breeding) plumage. (photo by Jeff Lewis)

So, we settle on Common Tern. Is it an adult, or a immature? Non-breeding adults can resemble immatures in many ways; both have the dark carpal bar on the upper wing, and both have a white forehead. Typically, non-breeding adults will have a more uniform gray color to the back and upperwing, while immatures have a more scalloped pattern. This is difficult to appreciate in a bedraggled specimen like Bird A, however. What I am going by is partly the tail, which is not as long and forked as I might expect in an adult, and also the time of year. On average, adults should still be mostly in their breeding plumage in summer, and breeding birds have red legs, red bills and black foreheads. Bird A has no trace of red on either legs or bill, so I think this bird is an immature. And Wouter agrees! Which always helps me rest easier.

I want to state for the record that I am terrible at identifying live terns. But I hope my dead bird i.d. skill has served me well in this instance.

 

Bird B: underside of wing.

Bird B: underside of wing.

Now, for Bird B, which is really quite close to a lost cause in terms of identification. It’s a wing, but not a complete wing. All the secondaries and secondary coverts are gone. So all we know is that the primaries are dark, and the underwing (at least out at the wingtip) is quite strikingly white. This is not a common feature and few species of birds have it. The ones that came to mind for me when I saw that white were shearwaters and loons. Wouter also thought loon. Here’s a link to an image of the underside of a Red-throated Loon wing, by way of example. The wing chord on my Bird B is pretty small for any of the loons though, at a mere 21 cm.

When I really thought about shearwaters, I decided I’m not so enthusiastic about them. Their underwings are generally white, but seem to kind of fade to gray well short of the primaries, unlike this bird.

A lot of alcids have white underwings, but they have black upperwings, not the dark grayish brown that this bird has.

Then there’s Dennis’ guess: a Red-necked Grebe. Grebe also crossed my mind, and drives home one of the great difficulties in this case of Bird B. Lacking any of the secondaries or the rest of the wing, we cannot assess many of the characteristics that would argue for grebes: a white speculum or a white patch near the shoulder; a pale trailing edge to some of the secondaries; or other such features. We can only speculate regarding the speculum (groan). Take a look at the Red-necked Grebe’s underwing and see what you think. I think it’s a real contender.

In short, I’m not sure.

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